PERF/API: Remove ordered requirement in Categorical.searchsorted (pandas-dev#21686)

Author: topper-123

doc/source/whatsnew/v1.0.0.rst

@@ -174,7 +174,7 @@ Categorical
 
 - Added test to assert the :func:`fillna` raises the correct ValueError message when the value isn't a value from categories (:issue:`13628`)
 - Bug in :meth:`Categorical.astype` where ``NaN`` values were handled incorrectly when casting to int (:issue:`28406`)
--
+- :meth:`Categorical.searchsorted` and :meth:`CategoricalIndex.searchsorted` now work on unordered categoricals also (:issue:`21667`)
 -
 
 

pandas/core/arrays/categorical.py

@@ -1399,13 +1399,6 @@ def memory_usage(self, deep=False):
     @Substitution(klass="Categorical")
     @Appender(_shared_docs["searchsorted"])
     def searchsorted(self, value, side="left", sorter=None):
-        if not self.ordered:
-            raise ValueError(
-                "Categorical not ordered\nyou can use "
-                ".as_ordered() to change the Categorical to an "
-                "ordered one"
-            )
-
         from pandas.core.series import Series
 
         codes = _get_codes_for_values(Series(value).values, self.categories)

pandas/core/base.py

@@ -1515,6 +1515,12 @@ def factorize(self, sort=False, na_sentinel=-1):
         corresponding elements in `value` were inserted before the indices,
         the order of `self` would be preserved.
 
+        .. note::
+
+            The %(klass)s *must* be monotonically sorted, otherwise
+            wrong locations will likely be returned. Pandas does *not*
+            check this for you.
+
         Parameters
         ----------
         value : array_like
@@ -1540,6 +1546,7 @@ def factorize(self, sort=False, na_sentinel=-1):
 
         See Also
         --------
+        sort_values
         numpy.searchsorted
 
         Notes
@@ -1578,6 +1585,13 @@ def factorize(self, sort=False, na_sentinel=-1):
 
         >>> x.searchsorted(['bread'], side='right')
         array([3])
+
+        If the values are not monotonically sorted, wrong locations
+        may be returned:
+
+        >>> x = pd.Series([2, 1, 3])
+        >>> x.searchsorted(1)
+        0  # wrong result, correct would be 1
         """
 
     @Substitution(klass="Index")

pandas/tests/arrays/categorical/test_analytics.py

@@ -78,65 +78,52 @@ def test_mode(self, values, categories, exp_mode):
         exp = Categorical(exp_mode, categories=categories, ordered=True)
         tm.assert_categorical_equal(res, exp)
 
-    def test_searchsorted(self):
+    def test_searchsorted(self, ordered_fixture):
         # https://github.com/pandas-dev/pandas/issues/8420
         # https://github.com/pandas-dev/pandas/issues/14522
 
-        c1 = Categorical(
-            ["cheese", "milk", "apple", "bread", "bread"],
-            categories=["cheese", "milk", "apple", "bread"],
-            ordered=True,
-        )
-        s1 = Series(c1)
-        c2 = Categorical(
+        cat = Categorical(
             ["cheese", "milk", "apple", "bread", "bread"],
             categories=["cheese", "milk", "apple", "bread"],
-            ordered=False,
+            ordered=ordered_fixture,
         )
-        s2 = Series(c2)
+        ser = Series(cat)
 
         # Searching for single item argument, side='left' (default)
-        res_cat = c1.searchsorted("apple")
+        res_cat = cat.searchsorted("apple")
         assert res_cat == 2
         assert is_scalar(res_cat)
 
-        res_ser = s1.searchsorted("apple")
+        res_ser = ser.searchsorted("apple")
         assert res_ser == 2
         assert is_scalar(res_ser)
 
         # Searching for single item array, side='left' (default)
-        res_cat = c1.searchsorted(["bread"])
-        res_ser = s1.searchsorted(["bread"])
+        res_cat = cat.searchsorted(["bread"])
+        res_ser = ser.searchsorted(["bread"])
         exp = np.array([3], dtype=np.intp)
         tm.assert_numpy_array_equal(res_cat, exp)
         tm.assert_numpy_array_equal(res_ser, exp)
 
         # Searching for several items array, side='right'
-        res_cat = c1.searchsorted(["apple", "bread"], side="right")
-        res_ser = s1.searchsorted(["apple", "bread"], side="right")
+        res_cat = cat.searchsorted(["apple", "bread"], side="right")
+        res_ser = ser.searchsorted(["apple", "bread"], side="right")
         exp = np.array([3, 5], dtype=np.intp)
         tm.assert_numpy_array_equal(res_cat, exp)
         tm.assert_numpy_array_equal(res_ser, exp)
 
         # Searching for a single value that is not from the Categorical
         msg = r"Value\(s\) to be inserted must be in categories"
         with pytest.raises(KeyError, match=msg):
-            c1.searchsorted("cucumber")
+            cat.searchsorted("cucumber")
         with pytest.raises(KeyError, match=msg):
-            s1.searchsorted("cucumber")
+            ser.searchsorted("cucumber")
 
         # Searching for multiple values one of each is not from the Categorical
         with pytest.raises(KeyError, match=msg):
-            c1.searchsorted(["bread", "cucumber"])
+            cat.searchsorted(["bread", "cucumber"])
         with pytest.raises(KeyError, match=msg):
-            s1.searchsorted(["bread", "cucumber"])
-
-        # searchsorted call for unordered Categorical
-        msg = "Categorical not ordered"
-        with pytest.raises(ValueError, match=msg):
-            c2.searchsorted("apple")
-        with pytest.raises(ValueError, match=msg):
-            s2.searchsorted("apple")
+            ser.searchsorted(["bread", "cucumber"])
 
     def test_unique(self):
         # categories are reordered based on value when ordered=False